Fix #1157: Implement versioned library naming

[lht]

Green on all bots.
#1157

[brson]

This looks good to me so far. A few comments:

• This seems to generate both the libfoo.so and libfoo-hash-vers.so. Instead, can we just generate the hashed filename and call 'touch libfoo.so' in the build rules to satisfy make?
• We need to update 'make clean' to delete these hash-based libraries
• We need to update the snapshot scripts to handle hash-based libraries
• I'm worried about what happens when we've globbed-copied these libraries around for a while. It seems like we'll eventually end up with multiple libraries with the same version but different hashes.

[lht]

Thanks for reviewing! I'll update and resent later!

[lht]

Updated with all comments addressed. (except the last one)

(Looks like github dislike rebasing of pull requests. All inline comments given to the previous pull request are no longer listed on this page. Or, do we prefer close old pull request and open new one in such case?)

I've also removed the change about including hashes of dependencies from my previous pull request. Because there is not discrimination about direct and indirect dependencies in the current implementation. I would prefer only include hashes of direct dependent crates, and encoding hash and hashes of direct dependencies into crates metadata. There are a few other details I'm not clear of. So I think it's better to start a new series on that.

One thing in particular is what should be hashed? According to comments about build_link_meta:

 *  - Define CMETA as all the non-name, non-vers exported meta tags in the
 *    crate (in sorted order).
 *
 *  - Define CMH as hash(CMETA).
 *

If version is not hashed, I wonder how can it be transited across the dependency DAG.

[graydon]

Version is not hashed in order to permit backward compatible libraries to be upgraded without causing all dependant libraries to be recompiled. If the user wishes to force that they can change any extra link metadata tags.

This is a compromise between the desire to control and accurately reflect dependency, and the desire to do upgrades in the field the way os vendors prefer, using major.minor style versioning.

[lht]

Another issue, for crates with different names and without extra meta data, they may end up the same hash. I think it would be good to use hash as a unique id when resolving crate dependencies. Actually, there are more than one FIXMEs about replacing the cnum with hash.

Is there any background about not including crate name in hash?

[brson]

This looks really good.

[graydon]

Pushed to master, working through first snapshot now. Here's hoping!

[graydon]

Appears to have stuck with a couple little hacks to make the snapshots load right. Further work on getting version numbers to make sense given OS conventions will be done on branch. Thanks, this is great!